Active and passive millimeter-wave imaging systems have been demonstrated to have the capability to penetrate clothing and produce images of the person underneath the clothing, together with a wide variety of concealed threats including explosives, handguns, and knives. Examples of such systems are found in the following references. The entire text of these references, and all other papers, publications, patents, or other written materials disclosed herein are hereby incorporated into this specification in their entirety by this reference.    1. Sheen, D. M., D. L. McMakin, and T. E. Hall, Three-dimensional millimeter-wave imaging for concealed weapon detection. IEEE Transactions on Microwave Theory and Techniques, 2001. 49(9): p. 1581-92.    2. Sheen, D. M., et al., Concealed explosive detection on personnel using a wideband holographic millimeter-wave imaging system. Proceedings of the SPIE—AEROSENSE Aerospace/Defense Sensing and Controls, 1996. 2755: p. 503-13.    3. McMakin, D. L., et al. Detection of Concealed Weapons and Explosives on Personnel Using a Wide-band Holographic Millimeter-wave Imaging System. in American Defense Preparedness Association Security Technology Division Joint Security Technology Symposium. 1996. Williamsburg, Va.    4. McMakin, D. L., et al., Wideband, millimeter-wave, holographic weapons surveillance system. Proceedings of the SPIE—EUROPTO European symposium on optics for environmental and public safety, 1995. 2511: p. 131-141.    5. Sinclair, G. N., et al., Passive millimeter-wave imaging in security scanning. Proc. SPIE, 2000. 4032: p. 40-45.    6. Sheen, D. M., D. L. McMakin, and T. E. Hall, Combined illumination cylindrical millimeter-wave imaging technique for concealed weapon detection. Proceedings of the SPIE—Aerosense 2000: Passive Millimeter-wave Imaging Technology IV, 2000.4032.    7. Sheen, D. M., D. L. McMakin, and T. E. Hall, Cylindrical millimeter-wave imaging technique for concealed weapon detection. Proceedings of the SPIE—26th AIPR Workshop:Exploiting new image sources and sensors, 1997. 3240: p. 242-250.    8. McMakin, D. L. and D. M. Sheen. Millimeter-wave high-resolution holographic surveillance systems. in AAAE Airport Security Technology Conference. 1994. Atlantic City, N.J.: AAAE.    9. McMakin, D. L., et al., Cylindrical holographic imaging system privacy algorithm final report. 1999, Pacific Northwest National Laboratory: Richland, Wash.    10. Keller, P. E., et al., Privacy algorithm for cylindrical holographic weapons surveillance system. IEEE Aerospace and Electronic Systems Magazine, 2000. 15(2): p. 17-24.    11. Michelson, D. G. and I. G. Cumming, A calibration algorithm for circular polarimetric radars. Journal of Electromagnetic Waves and Applications, 1997. 11: p. 659-674.    12. Yueh, S. and J. A. Kong, Calibration of polarimetric radars using in-scene reflectors. Journal of Electromagnetic Waves and Applications, 1990. 4(1): p. 27-48.    13. Fujita, M., et al., Polarimetric calibration of the SIR-C C-Band channel using active radar calibrators and polarization selective dihedrals. IEEE Transactions on Geoscience and Remote Sensing, 1998. 36(6): p. 1872-1878.    14. U.S. Pat. No. 5,859,609 “Real-Time Wideband Cylindrical Holographic System” issued Jan. 12, 1999 to Sheen et al.    15. U.S. Pat. No. 6,507,309 “Interrogation of an Object for Dimensional and Topographical Information” issued Jan. 14, 2003 to McMakin et al.    16. U.S. Pat. No. 6,703,964 “Interrogation of an Object for Dimensional and Topographical Information” issued Mar. 9, 2004 to McMakin et al.    17. U.S. patent application Ser. No. 10/607,552, “Concealed Object Detection,” filed Jun. 26, 2003    18. U.S. patent application Ser. No. 10/697,848, “Detecting Concealed Objects at a Checkpoint,” filed Oct. 30, 2003.
Active millimeter-wave imaging systems operate by illuminating the target with a diverging millimeter-wave beam and recording the amplitude and phase of the scattered signal over a wide frequency bandwidth. Highly efficient Fast Fourier Transform (FFT) based image reconstruction algorithms can then mathematically focus, or reconstruct, a three-dimensional image of the target as described in Sheen, D. M., D. L. McMakin, and T. E. Hall, Three-dimensional millimeter-wave imaging for concealed weapon detection. IEEE Transactions on Microwave Theory and Techniques, 2001. 49(9): p. 1581-92. Millimeter-waves can readily penetrate common clothing materials and are reflected from the human body and any concealed items, thus allowing an imaging system to reveal concealed items. Passive millimeter-wave imaging systems operate using the natural millimeter-wave emission from the body and any concealed items. These systems use lenses or reflectors to focus the image, and rely on temperature and/or emissivity contrast to form images of the body along with any concealed items. In indoor environments passive systems often have low thermal contrast, however, active illumination has been demonstrated to improve the performance of these systems. Active millimeter-wave imaging systems have several advantages over passive systems including elimination of bulky lenses/reflectors, high signal-to-noise ratio operation, and high contrast for detection of concealed items. In addition to millimeter-wave imaging systems, backscatter x-ray systems have also been developed for personnel screening. These systems can be very effective, however, they are bulky and may not be well-received by the public due to their use of ionizing radiation (even though they operate at low x-ray levels).
Active, wideband, millimeter-wave imaging systems have been developed for personnel screening applications. These systems utilize electronically controlled, sequentially switched, linear arrays of wideband antennas to scan one axis of a two-dimensional aperture. A high-speed linear mechanical scanner is then used to scan the other aperture axis. The microwave or millimeter-wave transceiver is coupled to the antenna array using a network of microwave/millimeter-wave switches. Amplitude and phase reflection data from the transceiver are gathered over a wide frequency bandwidth and sampled over the planar aperture. These data are then focused or reconstructed using a wideband, three-dimensional, image reconstruction algorithms. The resolution of the resulting images is diffraction-limited, i.e. it is limited only by the wavelength of the system, aperture size, and range to the target and is not reduced by the reconstruction process. Preferred algorithms make extensive use of one, two, and three-dimensional FFT's and are highly efficient. Imaging systems utilizing a planar, rectlinear aperture are restricted to a single view of the target. To overcome this limitation, a cylindrical imaging system has been developed. This system utilizes a vertical linear array that has its antennas directed inward and is electronically sequenced in the vertical direction and mechanically scanned around the person being screened. Data from this system can be reconstructed over many views of the target creating an animation of the imaging results in which the person's image rotates.
All imaging systems proposed for personnel screening typically produce image artifacts, created by the detection of signals that have been reflected more than one time (multipath) by the item being imaged. These artifacts are undesirable because they distract from the underlying item being imaged, such as concealed objects. Accordingly, there is a need for new imaging techniques that highlight concealed objects, and/or suppress multipath artifacts in the images.